Add workspacefolder option for mcp server config

[wss29]

When configuring the MCP server, such as with @modelcontextprotocol/server-filesystem, it would be more convenient to include the current workspace as an argument.

Originally raised in 14679

[JonasHelming]

The issue is that we have to pass the allowed directories at start-up. The user can change the workspace after we started the MCP server. It would be better if the the file system MCP server would work with roots, which I requested: modelcontextprotocol/servers#401

[JonasHelming]

Additionally, this option is for the filesystem server only, as allowedDirectories is a custom argument.
So as useful as this would be, I would rather wait until modelcontextprotocol/servers#401 is fixed and then support an option "addWorkspaceAsRoot" which is generic for all MCP servers that support roots.